The present invention relates to an air-fuel ratio control system for an engine of a motor vehicle, and more particularly for an engine supplied with lean mixture.
A lean mixture engine is disclosed, for example, in Japanese Patent Laid Open 58-48749.
The lean mixture engine operates on lean mixture at light and middle load and on stoichiometry mixture at heavy load. A feedback air-fuel ratio control is provided for supplying the air-fuel mixture at large air-fuel ratio (lean mixture) or stoichiometric air-fuel ratio in accordance with engine operating conditions.
The feedback control system is provided with a lean mixture sensor for sensing the oxygen concentration of the exhaust gas of the engine, the output voltage of which is proportional to the oxygen concentration. In a fuel injection system for the lean mixture engine, a plurality of desired air-fuel ratios are stored in a look-up table in accordance with engine operating conditions. A feedback signal from the lean mixture sensor is compared with the desired air-fuel ratio AFd to produce an error signal. A feedback coefficient K.sub.FB for fuel injection is calculated based on the error signal. On the other hand, an air-fuel ratio coefficient K.sub.AF based on engine operating conditions and a miscellaneous coefficient COEF including a plurality of coefficients based on various operating conditions such as coolant temperature, intake air temperature and other are calculated based on the desired air-fuel ratio AFd.
Fuel injections time TI of injected fuel is calculated as follows. EQU TI=K.times.K.sub.AF .times.K.sub.FB .times.Q/N.times.COEF (1)
where
K is a correcting coefficient, where PA1 Q is intake air flow rate, and PA1 N is engine speed.
By injecting fuel during the calculated time, the air-fuel ratio is controlled to the desired air-fuel ratio.
Referring to FIG. 6, when the desired air-fuel ratio AFd varies from one value to another value dependent on the changing of engine operating condition at a time t.sub.1, the control of the air-fuel ratio is delayed in spite of the immediate change of the feedback coefficient K.sub.FB, because of inherent time delay. As a result, the control system oscillates, so that the actual air fuel ratio AFa oscillates as shown in FIG. 6.
In order to prevent this hunting, for example, Japanese Patent Laid Open 58-59330 discloses a system in which, when a desired air-fuel ratio changes, a correcting coefficient is increased until the output of a lean mixture sensor exceeds a set value.